Chromatography is a technique for separating mixtures. A mixture may comprise two or more components, for example, two or more different molecules or chemical compounds. The mixture may further comprise a solvent for solubilizing the two or more components (solutes). In chromatography, the dissolved mixture is known as a mobile phase. Note that the term “phase,” as used herein in the terms mobile phase and stationary phase, means a physically distinctive form of a substance. A mobile phase signifies movement or mobility of the substance (i.e., the dissolved mixture). A stationary phase signifies that the substance of the stationary phase is not mobile. Chromatography involves passing the mobile phase through a stationary phase. The stationary phase is part of the device performing the chromatography (chromatography device) and separates the analyte to be measured from the other components in the mixture, based on differential partitioning between the mobile phase and the stationary phase. Components in a sample mixture injected into the chromatography stationary phase travel different distances based on how strongly they interact with the stationary phase. Retention time of a single molecule or other component inside a chromatography channel is indicative of the properties of the molecule or other component such as hydrophobicity or size. Further, the specific retardation time of a particular component passing through a chromatography channel can be used to identify the component.
Chromatographic technologies are essential to life sciences research, as they are useful in both the purification of pharmaceutical products and in analytical assays. Detectors coupled to the chromatography device are able to produce a signal that is plotted against retention time to produce elution peaks. Analysis of such peaks allows determinations to be made about the makeup of the mixture which is injected into the chromatography device. Additionally, since different compounds have different retention times within the stationary phase of a chromatography device, these compounds and molecules may be separated by separate collection of the effluent during different peak elutions.
Chromatographic techniques vary by stationary phase structure, shape and orientation. Column chromatography may involve a packed column, that is, a tube packed with the stationary phase which is often a resin or bead-matrix. Alternately, column chromatography may involve an open column, that is, a tube having a stationary phase that is bonded (e.g., covalently bonded) to the inside wall of the tube and having an unobstructed opening through the length of the tube. Differences in rates of movement of components through the stationary phase are according to different retention times of the different types of components.
Planar chromatography is a separation technique in which the stationary phase is present on a plane. An analysis of the mixture is available through the observance of different rates of movement of the different components present in the mobile phase through the stationary phase of the planar chromatography device. Thin-layer and paper chromatography is widely practiced in various applications. In planar chromatography, the stationary phase may be either special chromatography paper or a thin, flat layer of an inert substrate such as silica gel, alumina, or cellulose. In paper chromatography, the sample may be applied to a polar celluloid substance. A solvent is applied to the paper. As the solvent rises through the paper, it encounters the sample mixture. The components in the sample mixture travel up the paper with the solvent, with those components that are less-polar traveling further. Different states of the mobile phases of chromatographic processes are also known with both gaseous and liquid mobile phases being used.